1. Field of the Invention
The present invention relates to a circuit for receiving a push-button signal and identifying a dialed number based on the received signal, and to a method of detecting such a push-button signal.
2. Description of the Related Art
In analog telephone communications, push-button signal (hereinafter abbreviated as PB signal) is used as a selection signal whereby a telephone terminal specifies a target of connection. In such communications using the PB signal, one of low-frequency identification signals with frequencies of 697 Hz, 770 Hz, 852 Hz and 941 Hz is combined with one of high-frequency identification signals with frequencies of 1209 Hz, 1336 Hz, 1477 Hz and 1633 Hz so that based on a total of 16 different signals, a dialed number can be identified. A low-frequency signal and a high-frequency signal, which are used in combination, are continuously sent out for a fixed time or longer in response to a single dialing operation. Also, between dealing operations, there is provided a time period which is equal to or longer than a second fixed time called minimum cause and in which no signal is present.
A PB signal receiving circuit receives the combination of the low- and high-frequency signals, and judges that the input signal is a valid selection signal if the signal has been continuously received over the first fixed time. Also, if reception of the signal discontinues for the second fixed time or longer, the PB signal receiving circuit regards such a signal discontinuance time as the minimum pause, and judges a signal with the same frequency received thereafter to be a different selection signal.
FIG. 6 exemplifies a schematic configuration of a conventional PB signal receiving circuit generally used.
A PB signal receiving circuit 30 shown in FIG. 6 comprises a filter 31 for removing a dial-tone signal from the input signal, filters 32 and 33 for separating the input signal into low- and high-frequency signals, respectively, frequency detection circuits 34 and 35 for the low- and high-frequency signals, respectively, a signal determination circuit 36 for identifying a dialed number based on the detected frequencies, a control circuit 37 for determining valid length as the identification signal, and an output circuit 38 for outputting number data with a valid length.
The filter 31 removes, from the input signal, the dial-tone signal which is an audible signal with a frequency lower than the low-frequency signals and which prompts the user of a telephone terminal to send a selection signal. The filters 32 and 33 selectively pass only low- and high-frequency bands, respectively, of the signal output from he filter 31. The frequency detection circuits 34 and 35 detect the frequencies of the respective input signals. If both of the frequency detection circuits 34 and 35 detect valid low and high frequencies, respectively, the signal determination circuit 36 recognizes detection of low- and high-frequency identification signals and outputs a detection signal to the control circuit 37. Also, the signal determination circuit identifies a dialed number based on the combination of the identification signals and outputs number data indicative of the identified number to the output circuit 38. The control circuit 37 monitors the continuance time and discontinuance time of the input detection signal to determine valid length as the identification signal, and outputs a validity signal EN indicating the length. The output circuit 38 outputs number data having a length based on the validity signal EN from the control circuit 37, as 4-bit data D31, D32, D33 and D34, for example.
Referring now to FIG. 7 which is a timing chart showing signals appearing in various parts of the above PB signal receiving circuit 3C, the operation of the receiving circuit 30 will be described.
A input signal S41 input to the PB signal receiving circuit 30 includes the dial tone, low-frequency signal, high-frequency signal, etc. The filter 31 outputs a signal S42 from which the dial tone has been removed, and the filters 32 and 33 output signals S43 and S44 containing only components of the low- and high-frequency ranges, respectively. The frequency detection circuits 34 and 35 output identification signals S45 and S46, which correspond to their respective assigned low and high frequencies, to the signal determination circuit 36. The signal determination circuit 36 outputs a detection signal S47 corresponding to the PB signal, which is derived as a logical product of the identification signals S45 and S46, to the control circuit 37.
The control circuit 37 monitors the continuance time and discontinuance time of the input of the detection signal S47, and outputs the validity signal EN indicating a valid length as the selection signal. Specifically, the control circuit 37 starts to count the input continuance time at timing T21 at which The detection signal S47 turns to the H level, for example. If the detection signal S47 remains at the H level up to timing T22 over a preset continuance criterion time Ton, the control circuit judges that the detection signal S47 is a valid selection signal, and turns the validity signal EN to the H level. At timing T23 at which the detection signal S47 turns to the L level, the control circuit starts to count the input discontinuance time. However, since the detection signal S47 again turns to the H level at timing T24 before a preset discontinuance criterion time Toff elapses, the control circuit does not judge that the selection signal has discontinued, and holds the validity signal EN at the H level. The control circuit again starts to count the discontinuance time at timing T25 at which the detection signal S47 turns to the L Level, and at timing T26 after a lapse of the discontinuance criterion time Toff for which the detection signal remained at the L level, the control circuit judges that the selection signal has discontinued, and turns the validity signal EN to the L level. The output circuit 38 outputs the number data D31 to D34 having a valid data length based on the rise and fall timings of the validity signal EN.
FIG. 8 shows frequency detection characteristics of the PB signal receiving circuit 30.
As shown in FIG. 8, in the PB signal receiving circuit 30, allowable frequencies fah and fal are set respectively as upper and lower limits of frequency for allowing signal reception, with respect to a nominal frequency fo of each identification signal, and a signal with a frequency falling within the range between the allowable frequencies fah and fal is received without fail. Also, forbidden frequencies fph and fpl are set so as to be separated from the allowable frequency range, and a signal with a frequency higher than the forbidden frequency fph and a signal with a frequency lower than the forbidden frequency fpl are not accepted. A region between the allowable frequency fah and the forbidden frequency fph and a region between the allowable frequency fal and the forbidden frequency fpl each constitute an uncertainty region in which whether signal is received or not is uncertain. In order to prevent erroneous operation, therefore, the range between the allowable frequencies fah and fal should desirably be narrowed, and also the values of the forbidden frequencies fph and fpl should desirably be as close to the respective allowable frequencies fah and fal as possible.
Provided a frequency deviation in which reception is allowed is da and a frequency deviation in which reception is forbidden is dp (da<dp), the nominal frequency fo, the allowable frequencies fah and fal, and the forbidden frequencies fph and fpl are in the relationships indicated by the following equations (1), (2), (3) and (4):fah=(1+da)×fo  (1)fal=(1−da)×fo  (2)fph=(1+dp)×fo  (3)fpl=(1−dp)×fo  (4)
Meanwhile, a method of detecting frequency in the frequency detection circuits 34 and 35 includes a method of extracting a signal with a specified frequency by using a filter and a method of measuring period. Of these methods, the period measuring method is more often used because of simplicity of circuit configuration and hence higher economical efficiency.
To measure the period, an interval between time points at which the input signal level crosses a certain threshold is measured, for example. In cases where the input signal includes noise whose frequency falls within the passband of the filter 32 or 33, however, such noise can vary the threshold crossing timing, thus causing jitter. FIG. 9 illustrates noise-induced period variations during the period measurement.
In FIG. 9, the threshold for frequency detection is set at 0 V. Also, in the figure, a signal S51 shows an example of input signal waveform including no noise, and signals S52 and S53 individually show the input signal on which noise is superimposed in a range such that the threshold crossing timing is varied by +Δt at the maximum. Provided the peak level of the signal S51 is S, the noise level with respect to the signal S51 is N, and the period of the signal S51 is T, thensin(2π×(Δt/T))=N/S  (5)Δt=T×sin−1(N/S)/2π  (6)
As seen from equation (6) above, the variation Δt in the threshold crossing point is dependent on the ratio of the noise level N to peak level S of the signal S51. Also, because of inclusion of noise, a detected period varies in the range from a minimum value Tmin (=T−2Δt) to a maximum value Tmax (=T+2Δt), as shown in FIG. 9. Jitter J caused in this case is indicated by equation (7) below.J=2Δt/T=sin−1(N/S)/π  (7)
FIG. 10 is a graph showing the relationship between the amplitude ratio N/S of noise to signal and jitter J, based on equation (7), and FIG. 11 illustrates the influence of jitter upon the frequency detection characteristics.
As shown in FIG. 10, the amount of jitter J increases with increase in the amplitude ratio N/S of noise to signal. Also, as shown in FIG. 11, in the case of allowing the reception of noise in a range in which jitter J is caused, then it is necessary that the absolute value of the allowable frequency deviation for the frequency detection should be da+J, and that the absolute value of the forbidden frequency deviation should be dp−J. Thus, in the case where the frequency range in which noise is admitted at the time of frequency detection is broadened, the forbidden frequency deviation also should be enlarged. This, however, lowers the frequency detection accuracy often causes an error in determining the presence/absence of input signal.
Usually, ambient noise picked up by the microphone of a telephone terminal or noise caused by crosstalk of lines, etc. is superimposed on the signal input to the PB signal receiving circuit 30, and it is therefore desirable that the identification signal be detected while allowing inclusion of a certain degree of noise. However, in the conventional P3 signal receiving circuit 30, broadening the range of admitting noise entails an increase in the rate of occurrence of errors, as mentioned above. In order to enhance the accuracy in the frequency detection of the input signal while at the same time allowing inclusion of a certain degree of noise, a method may be adopted in which the frequency (period) detection cycle is set to multiple periods, not one period, of the input signal. If the detection cycle is set to n periods of the input signal, for example, the influence of jitter J on the frequency detection can be reduced to 1/n.
However, where frequency is detected over multiple periods of the input signal, a longer time is required before the detected frequency is established than in the case of the detection over a smaller number of periods, such as one period, so that error in determining the continuance and discontinuance of the input signal expands. FIG. 12 shows examples of frequency detection over multiple periods of the input signal, wherein (A) shows the input signal, (B) shows the frequency determination according to a first example of detection, (C) shows the output of the identification signal according to the first example of detection, (D) shows the frequency determination according to a second example of detection, and (E) shows the output of the identification signal according to the second example of detection.
FIG. 12 illustrates in time sequence how a low-frequency identification signal with a frequency of 697 Hz, for example, is detected, wherein the number n of periods of the input signal corresponding to one detection cycle is 20 (n=20). Thus, one detection cycle in which the detected frequency is established is 28.69 msec. Also, a maximum time for the detection over 20 periods of the input signal is set to 31.56 msec, which is 110% of one detection cycle. If 20 periods of the input signal exceed 31.56 msec, it is judged that the input signal is not valid as the 697-Hz identification signal, and the next period measurement of the input signal is started. Parts (C) and (E) of FIG. 12 illustrate the states of extraction of the identification signal S45 by the frequency detection circuit 34 appearing in FIG. 6.
As shown in (A) of FIG. 12, the input signal is actually input to the frequency detection circuit 34 from timing T32 to timing T40. In the first example of detection shown in (B) of FIG. 12, the period detection is started at timing T33 immediately after timing T32 of the input signal, and the detection time for 20 periods is counted. Then, at timing T35 after a lapse of 28.69 msec from the start of detection, reception of the 697-Hz Identification signal is detected, and the identification signal is extracted as shown in (C) of FIG. 12. The counting of the period detection time is again started thereafter for the next detection.
In the first example of detection, at timing T40 immediately after the detection is newly started at timing T39, the input signal discontinues. In this detection cycle, therefore, the input is judged invalid at timing T42 after a lapse of 31.56 ms, and the output of the identification signal is stopped, as shown in (C) of FIG. 12.
On the other hand, in the second example of detection shown in (D) of FIG. 12, the period detection is newly started at timing T31 and the period detection time is counted. However, since the input of the signal starts at timing T32 immediately after the start of the period detection, 20 periods of the input signal fail to be detected within 31.56 ms. Accordingly, the input signal is judged invalid at timing T34, and the counting of the period detection time is again started. As a result of the subsequent period detection terminating at timing T36, the identification signal is extracted as shown in (E) of FIG. 12.
Also, the detection is newly started at timing T38, and the input signal discontinues at timing T40 immediately before the counting of 20 periods is finished. In this detection cycle, therefore, the input is judged invalid at timing T41 after a lapse of 31.56 ms, and the output of the identification signal is stopped, as shown in (E) of FIG. 12.
As seen from the above, in the case where the detection cycle start timing is immediately after the reception start timing of the input signal, as in the first example of detection, the output start timing of the identification signal is earlier by a maximum of about one detection cycle, that is, about 28.69 msec, than in the case where the detection start timing is immediately before the reception start timing of the input signal, as in the second example of detection. Also, where the reception of the input signal ends immediately after the completion of counting for 20 periods, as in the first example of detection, the output stop timing of the identification signal is later by a maximum of about one detection cycle than in the case where the reception of the input signal ends immediately before the completion of counting for 20 periods, as in the second example of detection. Accordingly, the output timing of the identification signal is subject to an error of about two detection cycles at the maximum. Thus, in the case of the period detection over multiple periods of the input signal, the error in the output timing of the identification signal expands because of long detection cycle.
FIG. 13 illustrates exemplary cases where a short break occurs during the frequency detection over multiple periods of the input signal, wherein (A) shows the input signal, (B) shows the frequency determination according to a first example of detection, (C) shows the output of the identification signal according to the first example of detection, (D) shows the frequency determination according to a second example of detection, and (E) shows the output of the identification signal according to the second example of detection.
In FIG. 13, the identification signal with a frequency of 697 Hz is detected, n is set to 20 (n=20), so that one detection cycle is 28.69 msec, and a maximum allowable time for the detection of 20 periods is set to 31.56 msec, which is 110% of one detection cycle, as in FIG. 12.
As shown in (A) of FIG. 13, the signal actually input to the frequency detection circuit 34 undergoes a short break from timing T53 to timing T55. In the first example of detection shown in (B) of FIG. 13, the detection cycle starts anew at timing T52, and after the counting of signal continuance time is started, the input signal discontinues from timing T53 to timing T55. Since the period detection of the input signal is delayed because of the discontinuance time, 20 periods of the input signal fail to be detected within 31.56 ms; accordingly, the input signal is judged invalid at timing T56 and the output of the identification signal is stopped, as shown in (C) of FIG. 13. Subsequently, the counting of the period detection time is again started, and the output of the identification signal is started again at timing T58 as a result of the frequency detection.
On the other hand, in the second example of detection shown in (D) of FIG. 13, the period detection is newly started at timing T51, and the input signal discontinues at timing T53 before the input signal is received over 20 periods. At timing T54 within the discontinuance time, the maximum time 31.56 ms as counted from the start of the period detection elapses, so that the input is judged invalid, and the output of the identification signal is stopped, as shown in (E) of FIG. 13. Further, while the input signal is discontinued, the period detection is again started; therefore, also in the subsequent detection cycle, 20 periods of the input signal fail to be detected within 31.56 ms. Thus, the input signal is again judged invalid at timing T57, so that no identification signal is output. Subsequently, the frequency detection is newly started, and the output of the identification signal s started again at timing T59.
As seen from the above, in the case where the input signal is judged invalid at timing during the short break of the input signal, as in the second example of detection, the output stop time of the identification signal is prolonged for a maximum of about one detection cycle, compared with the first example of detection wherein the signal determination does not take place during the short break. Consequently, error in the output stop time of the identification signal expands with increase in the detection cycle.
As shown in the examples of FIGS. 12 and 13, in the case of detecting the period of the input signal, error in the extraction of the identification signal occurs depending on the relationships between the reception start and stop timings of the input signal and the start and end timings of the period detection, and this error becomes more and more noticeable as the detection cycle is prolonged. In the DB signal receiving circuit 30, the control circuit 37 detects the signal continuance time based on the extraction timing of the identification signal, to determine a valid length as the selection signal. With the method in which the period detection is performed over numerous periods of the input signal, therefore, error in the detection of the signal continuance time expands because of large error in the extraction timing of the identification signal, giving rise to a problem that erroneous operation occurs in relation to the output of number data.
Also, the input to the PB signal receiving circuit 30 very often includes, besides the original PB signal, ambient noise or voice picked up by the microphone of a telephone terminal as well as noise caused by crosstalk of lines, etc., as mentioned above, and such noises must not be detected as the identification signal. However, with the method in which the period detection is performed over numerous periods of the input signal, the input signal is judged to be an identification signal insofar as a predetermined number of periods of the input signal is detected within a prescribed detection time, even if the input frequency greatly varies during the detection cycle, so that erroneous detection of the identification signal is liable to occur.
On the other hand, in the case of the method wherein the period detection is performed over a fewer periods of the input signal, if the frequency range is broadened to admit noise at the time of frequency detection, then the forbidden frequency deviation needs to be enlarged, as mentioned above. This, however, lowers the frequency detection accuracy and also often causes erroneous operation in relation to the determination of the presence/absence of input signal.